1. Field of the Invention
The present invention relates to a polycrystalline silicon solar cell having a high efficiency and a method of fabricating the same, and, more particularly, to a method of forming a light-absorbing layer of a polycrystalline silicon solar cell, in which the light-absorbing layer is formed of non-polluted polycrystalline silicon using a metal-induced lateral crystallization (MILC) process, and grains of the polycrystalline silicon are vertically grown in the direction in which electrons and holes move using a metal-induced vertical crystallization (MIVC) process in which the polycrystalline silicon is used as a crystallization seed, so that the particle structure of the polycrystalline silicon is formed into a vertical columnar structure, with the result that the number of grain boundaries, acting as sites in which electrons and holes are recombined with each other, is minimized, thereby increasing the efficiency of a solar cell, to a high-efficiency polycrystalline silicon solar cell using the light-absorbing layer, and to a method of fabricating the high efficiency polycrystalline silicon solar cell.
2. Description of the Related Art
It has been 50 years or more since Belidusrnthdml Chapin, Fuller, Pearson et al. in the U.S. developed a solar cell in 1954. The solar cell had been chiefly used as an independent power source for remote places, such as a power source for space or the like, till the middle of the 1960's. However, owing to the oil crisis in the 1970's, the research and development of the solar cell has sought to produce cheap solar cells such that the solar cell can compete with commercial power supply systems. Thus, currently, the solar cell can be used as a power source for ground devices.
Solar photovoltaic power generation, which is a technology of directly converting solar energy into electric energy using a photovoltaic effect, is a future energy source which is being put to practical use for the first time due to the fact that solar energy is a permanent resource which does not cause thermal and environmental pollution and will not be exhausted so long as the sun exists.
Currently, wafer-type silicon solar cells are commercially used, and account for 80% or more of world solar cell market share. The price of the raw material of a silicon wafer is the most important among the factors determining the price of wafer-type silicon solar cells. A single-crystalline silicon (c-Si) solar cell is manufactured using a substrate having a thickness of 300˜400 μm, but, actually, the thickness of silicon which is sufficient to absorb light and generate electricity in a solar cell is 50 μm. However, when an ingot is cut into a silicon wafer, it is difficult to cut it to have a thickness of 300 μm or less, and the silicon wafer can be damaged in subsequent processes, so that it is impossible to manufacture a solar cell having a thickness of 300 μm or less.
In order to solve the above problem of such a wafer-type silicon solar cell, technologies of manufacturing a solar cell by depositing a silicon thin film on a cheap glass substrate have been earnestly researched from the 1980's. For example, there is an amorphous silicon (a-Si) thin film solar cell. This a-Si thin film solar cell is manufactured by vacuum-depositing an amorphous silicon thin film having a thickness of 1 μm or less on a cheap glass substrate. In this technology of manufacturing the a-Si thin film solar cell, its production cost can be decreased by reducing the thickness of the silicon constituting the a-Si thin film solar cell, but the a-Si thin film solar cell manufactured using this technology has basic problems in that its efficiency is lower than that of the single-crystalline silicon (c-Si) solar cell and in that its properties are deteriorated by the Staebler-Wronski' effect when it is exposed to light for a long period of time. The basic problems have not been completely solved yet although 20 years have passed since then. It is analyzed that the reason why this a-Si thin film solar cell has low efficiency and stability is that its silicon thin film, which is a light-absorbing layer, is amorphous.
Therefore, when a solar cell is manufactured using a crystalline silicon thin film, instead of an amorphous silicon thin film, as a light-absorbing layer, there are advantages in that the efficiency of the solar cell manufactured in this way can be increased to a level of that of a c-Si wafer solar cell and in that the production cost thereof can be decreased to a level of that of an a-Si thin film solar cell. Further, since a module using a glass substrate can be used as a window for a building, the module can be fabricated at relatively low cost. Moreover, since the module can be converted into a flexible module using a metal substrate, it can be used in various applications.
In order to decrease the production cost of a solar cell and to increase the efficiency thereof, a technology for forming a high-quality crystalline silicon thin film at a temperature of 500° C. or lower at which a cheap glass substrate does not deform must be first developed.
Methods of forming a polycrystalline silicon thin film at low temperature largely include two methods, that is, a method of directly forming a polycrystalline silicon thin film and a method of forming an amorphous silicon thin film and then converting amorphous silicon included in the amorphous silicon thin film into polycrystalline silicon through subsequent processes.
The former method of directly forming a polycrystalline silicon thin film is frequently performed using a chemical vapor deposition (CVD) method. In the chemical vapor deposition (CVD) method, a raw material, such as SiH4, is decomposed by the energy generated through plasma enhanced chemical vapor deposition (PECVD) or hot-wire chemical vapor deposition (HWCVD), and is then formed into a silicon thin film. However, in the case of the PECVD system, it is known that a process of forming a silicon thin film is excessively sensitive to the temperature of a substrate, and the silicon thin film formed using this PECVD system is very porous. Further, in the case of the HWCVD system, it is difficult to apply the HWCVD system to a large area system, this HWCVD system cannot be easily used to manufacture a solar cell.
The latter method of forming an amorphous silicon thin film and then crystallizing amorphous silicon thin film included in the amorphous silicon thin film includes a method of crystallizing amorphous silicon using a laser and a method of crystallizing amorphous silicon using a metal catalyst. Among them, the method of crystallizing amorphous silicon using a laser is not suitable for a process of manufacturing a solar cell, the process necessarily requiring a large area, because high-priced equipment is used.
A method of crystallizing amorphous silicon using a metal catalyst, such as nickel (Ni), palladium (Pd), gold (Au), aluminum (Al) or the like, is called metal induced crystallization (MIC). When this MIC is directly used to manufacture a solar cell, a large-sized polycrystalline silicon thin film can be formed, but the efficiency of a solar cell is decreased due to the contamination of metals included in the polycrystalline silicon, and thus it is difficult to manufacture a high-efficiency solar cell. Therefore, it is required to develop a crystallization technology which can minimize the metal contamination.
Hereinafter, the metal contamination causing the decrease in efficiency of a solar cell will be described in more detail with reference to the accompanying drawings.
FIG. 1 is a schematic sectional view showing a structure of a conventional solar cell.
Referring to FIG. 1, a conventional solar cell has a PIN structure including a P-type silicon layer 2, a light-absorbing layer 3 and an N-type silicon layer 4. A front electrode 1 is formed on the P-type silicon layer 2, a back electrode 5 is formed beneath the N-type silicon layer 4, and an anti-reflection film 6 is formed on the front electrode 1.
The principle of the solar cell is as follows. When light reaches the light-absorbing layer 3 via the anti-reflection film 6 and the P-type silicon layer 2, a pair of holes and a pair of electrons are generated in the light-absorbing layer 3, and the pair of holes and the pair of electrons are respectively moved to the P-type silicon layer 2 and N-type silicon layer 4 by the internal electric fields formed in the P-type silicon layer 2 and N-type silicon layer 4. In this case, holes accumulate in the P-type silicon layer 2, and electrons accumulate in the N-type silicon layer 4, so that an electric current is generated from the front electrode 1 and back electrode 5 connected to the respective P-type silicon layer 2 and N-type silicon layer 4, thereby constituting a cell.
Here, the efficiency of a cell is determined depending on the amount of holes and electrons accumulated in the cell when a constant amount of light reaches the cell.
When the amorphous silicon included in the light-absorbing layer 3 is crystallized using the MIC method, a large amount of catalytic metal is present in the light-absorbing layer 3. When these metal impurities (pollutants) in the light-absorbing layer 3 are increased, the pairs of holes and electrons generated by light are separated by the internal electric fields, and are thus recombined with the holes and electrons present in the metal impurities before they reach the P-type silicon layer 2 and N-type silicon layer 4, so that they are not accumulated in the P-type silicon layer 2 and N-type silicon layer 4.
Consequently, with the increase in the amount of metal impurities present in the light-absorbing layer 3, the recombination frequency of holes and electrons is increased, thus decreasing the charge accumulation rate of a cell, that is, the efficiency of a cell.
Recently, instead of the metal induced crystallization (MIC) in which metals directly induce the phase change of silicon, there has been proposed a method of crystallizing a silicon layer using a metal induced lateral crystallization (MILC) phenomenon in which silicide, which is a reaction product of metal and silicon, sequentially induces the crystallization of silicon while it being continuously laterally propagated (refer to S. W. Lee & S. K. Joo, IEEE Electron Device Letter, 17(4), p. 160, (1996)).
In particular, nickel (Ni), palladium (Pd) and the like are known as metals causing such an MILC phenomenon. When a silicon layer is crystallized using the MILC phenomenon, a silicide interface including metals is laterally moved due to the propagation of the phase change of silicon, thus crystallizing a silicon layer. Since the metal constituents used to induce the crystallization of the silicon layer barely remain in the silicon layer crystallized using the MILC phenomenon, there is an advantage in that the current leakage and other operational characteristics of the active layer of a thin film transistor (TFT) are not influenced. Further, since the crystallization of the silicon layer can be induced at a relatively low temperature of 300˜500° C. when the MILC phenomenon is used, there is an advantage in that several substrates can be simultaneously crystallized using a furnace without damaging the substrates.
FIG. 2 is a graph showing the amounts of catalytic metals included in silicon crystallized through a metal induced crystallization (MIC) method and a metal induced lateral crystallization (MILC) method.
Referring to FIG. 2, the amounts of catalytic metals included in silicon crystallized through a metal induced crystallization (MIC) method and a metal induced lateral crystallization (MILC) method were analyzed through Electron Probe Microanalysis (EPMA), and the results thereof are shown in FIG. 2. From FIG. 2, it can be seen that catalytic metals are barely detected in the silicon crystallized through the metal induced lateral crystallization (MILC) method.
Therefore, it will be expected that a high-efficiency polycrystalline silicon thin film solar cell can be formed on a cheap substrate at low temperature when a light-absorbing layer of a solar cell is prepared using a metal induced lateral crystallization (MILC) method.
Meanwhile, Korean Unexamined Patent Application Publication No. 2006-100806 discloses a thin film solar cell, in which a P-type silicon layer, which is an active layer having a P-N junction structure, has grains laterally grown through lateral crystallization, so that the flow of electrons is not obstructed by a grain boundary, with the result that the loss of electrons is reduced, thereby improving light efficiency.
Since sequential lateral crystallization (SLC) using a laser is used in this technology, problems related to the recombination of holes and electrons due to catalytic metals do not occur, but this technology is problematic in that large-area amorphous silicon cannot be efficiently crystallized and in that productivity is low compared to crystallization heat treatment using a furnace.
Conventional technologies of crystallizing amorphous silicon using a metal induced lateral crystallization (MILC) method has been chiefly used to improve the current leakage and other operational characteristics of a thin film transistor because an active layer of a thin film transistor, particularly, a channel region, is formed of crystalline silicon in which metal components remain in scarcity, and has not been used to manufacture a polycrystalline silicon thin film solar cell.
Meanwhile, in the case of a polycrystalline silicon solar cell, grain boundaries act as recombination defects, that is, recombination sites of electrons and holes, and thus it is required to control the grain boundaries.
FIG. 3 is a sectional view showing the recombination of holes and electrons through grain boundaries 3a when grains 3b are randomly grown in a conventional polycrystalline silicon light-absorbing layer 3. As shown in FIG. 3, a P-type silicon layer 2 is formed on the light-absorbing layer 3, and an N-type silicon layer 4 is formed beneath the light-absorbing layer 3.
When the light-absorbing layer 3 is formed by crystallizing amorphous silicon into polycrystalline silicon using a conventional crystallization method, as shown in FIG. 3, grains 3b are randomly grown, so that a plurality of grain boundaries 3a, acting as the recombination sites of electrons ({circle around (e)}−) and holes ({circle around (h)}+) generated in the light-absorbing layer 3, is formed, with the result that there is a problem in that the efficiency of a solar cell is decreased.